Mine roof bolts and anchors for securing such bolts within a preformed hole are common in the industry. The roof bolts are used to support the ceiling of a mine by compressing the ceiling upward into a self-supporting beam. Holes are predrilled within the ceiling and the bolts are inserted within the holes and in abutment with the ceiling. An anchor attached to the inserted end of the bolt secures the bolt within the hole while the bolt is tightened against the ceiling. The bolt may support beams or ceiling plates against the ceiling to distribute the compressive force over a larger area of ceiling surface. The bolt is able to support the ceiling only if the anchor remains secure within the hole. If the anchor slips, the bolt slips and compression is removed from the ceiling of the mine. Without the supporting compressive force of the bolt, the ceiling will eventually collapse. Therefore many attempts have been made to devise anchors which are impervious to slippage. Representative of such anchors which are in some manner similar in appearance to the present invention are those shown in U.S. Pat. Nos. 4,626,139; 4,147,458; 4,100,748; 2,950,602; 1,352,201 and 1,367,080 although some of these are not suitable for use as mine roof bolt anchors.
The '080 patent shows a bolt anchor which utilizes a set of flat hard metal washers which are stacked on a bolt in alternating sequence with a plurality of transversely bent washers of soft or deformable material. The washers are compressed to distend the bent washers into engagement with the bore hole walls. This compression is achieved by either inserting the bolt head into the bore hole in a non-rotable manner and compressing the washers by means of a nut threaded onto the bolt, thereby leaving a portion of the bolt protruding from the hole; by impaction of the washers into the hole and then threadably engaging the washers onto the bolt; or by placing a nut in the bottom of the hole in a non-rotating manner, filling the hole with washers and inserting a bolt to compress the washers. Each instance appears to require a bore hole full of washers and does not allow further travel of the bolt; furthermore, it appears that the bent washers on the ends of the alternating series would flatten first, thereby requiring motion of the flattened washer to enable flattening of the other washer. It is readily apparent that motion of a seated washer in an expansion anchor has deleterious effects.
The '201 patent shows an expansion anchor using a plurality of curved deformable washers mounted in facing pairs with the curvature of the adjacent washers being in opposition. These washers are compressed by means such as described with regard to the '080 patent. Again, it should be noted that the end washers are apparently going to engage the bore hole walls first and must slide along the hole in order to flatten the remaining washers.
The '602 patent shows an expansion anchor which uses split rings circumscribing the roof bolt. The split rings are of two different diameters and are designed to be compressed with one size within the other to engage both the bore hole and the roof bolt. A plurality of such anchor rings may be used with each set of anchor rings separated by a metal tube which transmits forces between the upper and lower rings. This device relies on an upper nut which must non-rotably engage the bore hole through the use of outward projections and a lower nut or strap which must advance with the bolt to compress the split rings vertically and thereby cause their lateral expansion. When the rings are fully seated the bolt cannot be advanced further due to the lower stop engaging the rings.
The '748 patent shows an expansion anchor which uses a wedge and bail support sleeve which move responsive to the rotation of a bolt such that the sleeve is compressed between the downwardly traveling wedge and the wall of the bore hole. The primary gripping force in this type anchor has been found to exist only in the region of the sleeve that is first expanded into engagement with the wall of the bore hole.
The '458 patent shows a roof pin anchor which utilizes a pair of resilient curved locking disc which are spaced apart by intermediate reinforcing spaces therebetween. The assemblage is forced upwardly into the bore hole. The discs are not compressed and engage the walls of the bore hole solely due to weight pulling downward on the bolt. The spacers provide a locus for the pivoting motion of the periphery of the subjacent and superjacent discs. More than one spacer element may separate the discs.
Despite the achievements and efforts involved is developing the abovementioned anchors, the problem of slippage remained. A significant improvement to mine roof bolts was made, however, with the invention set forth in '139 which utilizes a plurality of transversely bent elongated leaves which are compressed within a bail by the rotation of the roof bolt. The compression results in movement of the leaves toward a flattened position whereby the outer edges of the leaves engage the bore hole walls to anchor the bolt. Spacers are provided which evenly distribute the compressive forces among the leaves such that the leaves engage the bore wall simultaneously and with the same pressure. The bail is supported by an upper end of the mine roof bolt and depends therefrom on either side of the bolt. A nut threadably connected to the bolt is selectively urged downward against the leaves responsive to the rotation of the bolt to compress the leaves against the supporting bail and urge the outer edges of the leaves against the defining wall of the hole. As is shown herein, the present invention includes components similar in general description to selected components described in '139 and also includes additional components and improvements to those components set forth in '139 that, in combination, form a novel roof bolt anchor that will engage the defining face of a preformed hole and secure a mine roof bolt in non-slipping relation therein.